Automotive Painting

1. Definition and Process Status

Automotive painting is one of the "four major manufacturing processes" in vehicle production (stamping, welding/body-in-white, painting, and final assembly). It refers to the application of multiple protective and decorative coating layers onto the vehicle body's sheet metal surfaces. Its role goes well beyond aesthetics—over 60% of a vehicle body's anti-corrosion capability is determined by the paintwork, making it a critical factor in overall vehicle durability. The paint shop typically represents one of the largest capital investments and highest energy consumers in an automotive assembly plant, with its process standards directly dictating the vehicle's build quality.

2. Coating System and Functional Distribution

Modern passenger car body painting generally employs a four-layer composite coating system, with each layer serving a dedicated, indispensable role:

Coating Layer	Functional Positioning	Process Characteristics
Electrodeposition (ED / E-Coat)	Core anti-corrosion barrier for the body	Employs cathodic electrodeposition (CED) to uniformly coat internal and external cavities, offering superior adhesion and rust protection
Primer Surfacer	Blocks UV rays, fills minor surface imperfections, and provides a smooth base for the topcoat	Requires oven baking and curing after application
Basecoat	Imparts colour, metallic flake, and aesthetic finish to the body	Waterborne basecoat has become the eco-friendly industry standard
Clearcoat	Delivers scratch resistance, UV protection, gloss, and weather durability	The outermost protective layer that directly determines the long-term finish and service life of the paintwork

In the overall paint composition breakdown for passenger cars, the ED coat accounts for roughly 20%, primer surfacer about 10%, basecoat about 30%, clearcoat about 15%, and auxiliary coatings around 25%.

3. Key Process Steps

Pre-treatment and Electrodeposition (E-Coat): Following welding, the body-in-white (BIW) passes through a sequence of stages—including hot water rinse, pre-degreasing, degreasing, water rinsing, surface conditioning, phosphating, and deionised water rinsing—before entering the electrodeposition tank. During electrodeposition, parameters such as bath voltage, temperature, solid content, and pigment-to-binder ratio are precisely adjusted to achieve a target film thickness of around 20 μm. The electrocoated body is then transferred to an oven and cured at approximately 170–180°C for 20 minutes.

Primer Surfacer and Topcoat Application: Primer application is typically handled by automated robotic spray systems, after which the body enters a flash-off zone for 5–10 minutes for levelling, followed by oven baking at 140–150°C for roughly 30 minutes. The topcoat process (basecoat + clearcoat) employs a "wet-on-wet" method—after the basecoat is applied and given a brief 5–8 minute flash-off/pre-bake, the clearcoat is sprayed directly over it before the vehicle undergoes final curing in the topcoat baking oven.

4. Pre-treatment Anti-corrosion Process

Internal cavities and enclosed sections that are difficult for the ED coat to fully reach require supplementary anti-corrosion measures. Cavity wax is the standard solution—heated, liquefied anti-rust wax is injected into critical areas such as the floor pan, inner door panels, and wheel arches. Utilising residual heat along the production line to maintain fluidity, the wax sets to form a hydrophobic, anti-rust barrier over the e-coat. Typically, 0.5 to 2 kg of wax is applied per vehicle.

5. Development Trends

Automotive paint technology is shifting away from traditional methods towards greener, more energy-efficient, and intelligent solutions. The transition from solvent-based paints to waterborne and medium-to-high-solid formulations has become mainstream, drastically reducing volatile organic compound (VOC) emissions. Furthermore, modern paint shops are increasingly adopting dry-scrubber spray booths in place of conventional water-wash systems, boosting air recirculation efficiency from 60% to 95% and cutting energy consumption by over 25%. Today's state-of-the-art paint facilities can achieve VOC emissions of under 10 g/m², meeting world-class environmental standards.